Patient monitor and method of using same

ABSTRACT

A patient monitoring apparatus for monitoring and/or measuring a physiological characteristic of said patient. A user interface having an interior portion communicates with an airway of a user such that substantially all gas inhaled and exhaled by the user enters the interior portion of the user interface. At least one vent element is associated with the user interface and communicates the interior portion of the user interface with an ambient atmosphere outside the user interface. The vent element and user interface define a flow element across which a pressure differential is created during inhalation and exhalation. The pressure differential is the pressure difference between the pressure within the interior portion of the user interface and the pressure of the ambient atmosphere outside the user interface. A sensor communicates with the interior portion of the user interface and measures a fluid characteristic resulting from this pressure differential and outputs a first signal indicative of the measured fluid characteristic. The output from the sensor is used alone or in combination with the output from other sensors that detect other physiological characteristics to provide a variety of information about the patient.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation of U.S. Pat. application Ser. No. 09/030,221filed Feb. 25, 1998, now U.S. Pat. No. 6,017,315.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a patient monitor for monitoringand/or quantitatively measuring a physiological characteristic of thepatient, and, in particular, to an apparatus and method for monitoringand/or quantitatively measuring a physiological characteristic based, atleast in part, on a pressure differential between a pressure within auser interface and an ambient atmospheric pressure outside the userinterface.

2. Description of the Related Art

There are many situations in which it is necessary or desirable tomeasure a physiological characteristic of a patient, such ascharacteristics associated with respiration. Examples of characteristicsassociated with respiration include the patient's flow, inspiratoryperiod, expiratory period, tidal volume, inspiratory volume, expiratoryvolume, minute ventilation, respiratory rate, ventilatory period, andinspiration to expiration (I to E) ratio. It is also important in manysituations to identify still other characteristics associated withrespiration, such as identifying the start, end and duration of apatient's inspiratory phase and expiratory phase, as well as detectingpatient snoring. For example, when conducting a sleep study to diagnosesleep disorders or when conducting other pulmonary monitoring functions,it is common to measure the respiratory rate and/or the air flow to andfrom the patient. Distinguishing between inspiration and expiration isuseful, for example, in triggering a pressure support device thatprovides breathing gas to a patient.

There are several known techniques for monitoring patient breathing forthese purposes. A first conventional technique involves placing athermistor or thermocouple in or near the patient's airway so that thepatient's breath passes over the temperature sensing device. Breathinggas entering the patient has a temperature that is generally lower thanthe exhaled gas. The thermistor senses this temperature difference andoutputs a signal that can be used to distinguish between inspiration andexpiration.

A primary disadvantage of the thermistor or thermocouple air flowsensing technique is that these devices cannot quantitatively measurethe flow and/or volume of breathing gas delivered to and/or exhaled fromthe patient, because the signal from the sensor is a measure of airtemperature, not air flow or pressure. Typically, a thermistor air flowsensor is only used to differentiate between inspiration and expiration.Sensors that detect humidity have similar uses and similardisadvantages.

A second conventional technique for measuring the airflow to and from apatient is illustrated in FIG. 1 and involves placing a pneumotachsensor 30 in a breathing circuit 31 between a supply of breathing gas,such as a ventilator or pressure support device, and the patient'sairway. In a conventional pneumotach, the entire flow of breathing gasQ_(IN) is provided to a patient 32 from a pressure source 34.Conversely, all of the gas expelled from patient 32, passes throughpneumotach 30 so that during operation, there is a two-way flow of gasthrough pneumotach 30.

In its simplest form shown in FIG. 1, pneumotach 30 includes a flowelement 36 having an orifice 38 of a known size defined therein. Flowelement 36 provides a known resistance R to flow through the pneumotachso that a pressure differential ΔP exists across of flow element 36.More specifically, flow element 36 causes a first pressure P1 on a firstside of the flow element to be different than a second pressure P2 on asecond side of the flow element opposite the first side. Whether P1 isgreater than P2 or vice versa depends on the direction of flow throughthe pneumotach.

In a first type of conventional pneumotach, a major portion Q₁ of thetotal flow Q_(IN) of gas delivered to pneumotach 30 passes throughorifice 38. The pressure differential ΔP created by flow element 36causes a lesser portion Q₂ of the gas delivered to the pneumotach to bediverted through a bypass channel 40, which is connected to breathingcircuit 31 across flow element 36. An airflow sensor 42 in bypasschannel 40 measures the flow of gas therethrough. Because the area oforifice 38 and the area of bypass channel 40 are known and fixedrelative to one another, the amount of gas Q₂ flowing through bypasschannel 40 is a known fraction of the total gas flow Q_(IN) delivered topneumotach 30. Airflow sensor 42 quantitatively measures the amount ofgas Q₂ passing through bypass channel 40. Once this quantity is known,the total flow Q_(IN) of gas passing through pneumotach 30 can bedetermined.

In a second type of conventional pneumotach, a pressure sensor, ratherthan an airflow sensor, is provided in bypass channel 40. Gas does notpass through the pressure sensor. Instead, each side of a diaphragm inthe pressure sensor communicates with respective pressures P1 and P2 oneither side of flow element 36. The pressure sensor measures pressuredifferential ΔP across flow element 36. For example, for flow in thedirection illustrated in FIG. 1, pressure differential ΔP across flowelement 36 is P1-P2. Once pressure differential ΔP is known, the flowrate Q_(IN) of gas passing through pneumotach 30 can be determined usingthe equation, ΔP=RQ², where R is the known resistance of flow element36.

Another conventional pneumotach 44 is shown in FIG. 2. Pneumotach 44improves upon pneumotach 30 in FIG. 1 by providing a first linear flowelement 46 in place of flow element 36. First linear flow element 46functions in the same manner as flow element 36 by creating a pressuredifferential in breathing circuit 31. However, flow element 46 has aplurality of honey-comb like channels that extend in the direction ofgas flow to linearize the flow of gas through the pneumotach. Theprevious flow element 36 in FIG. 1 can create downstream turbulence thathinders the flow of gas through the bypass channel or causesfluctuations in the downstream pressure, thereby degrading the airflowor pressure differential signal output by sensor 42. Flow element 46solves this problem by providing a plurality of honeycomb-like channelshaving longitudinal axis parallel to the axis of the breathing circuit.The honeycomb channels ensure that the flow across the downstream portof the bypass channel is linear, i.e., non-turbulent.

To ensure that the flow of gas across the port in bypass channel 40upstream of flow element 46 is also linear, i.e., non-turbulent, otherlinear flow elements 48 and 50 are provided in the breathing circuitFlow elements 48 and 50 have the same honeycomb configuration as flowelement 46. Because gas can flow in both directions through pneumotach44, flow elements 48 and 50 are respectively located on each side offlow element 46 so that each entry port for bypass channel 40 isdownstream of one of these additional flow elements regardless of thedirection of flow through the pneumotach.

Although a pneumotach improves upon a theremistor in that itquantatively measures the flow and/or volume of gas passingtherethrough, it also has significant disadvantages. For example, apneumotach is relatively complicated and therefore difficult and costlyto manufacture. It is also difficult to clean and is relatively large.Because of its size, which is dictated by the need to measure thepressure differential or flow across the flow element in the breathingcircuit, it creates a relatively large amount of dead space in thepatient breathing circuit, which is not conducive to minimizingrebreathing of CO₂. Because of its complexity, a pneumotach may leak,and its operating capabilities can suffer as a result of heat andmoisture buildup.

A third type of conventional airflow meter, illustrated in FIG. 3, is anasal cannula airflow meter 52. Nasal cannula airflow meter 52 issimilar to a nasal oxygen cannula in that it includes a pair of ports 54and 56 that insert into nares 58 and 60 of the user. A hollow tubing 62carries a fraction of the total amount of breathing gas to a sensor,such as an airflow or pressure sensor, If the total area of the user'snares relative to the total area of the ports 54 and 56 is known, thenasal cannula airflow meter can provide a quantitative measure of thepatient airflow.

However, bececse the total area of each user's nares can vary fromperson to person, a commonly sized nasal cannula airflow meter cannotprovide an accurate, quantitative measure of the airflow for all users.If two people have different sized nasal openings, the fraction of theexhaled air that is being delivered to the ports of the nasal cannulacannot be known for both users. For example, a first user may deliver30% of the exhaled gas to the ports of the nasal cannula, while a seconduser may deliver only 10% of the exhaled to the same sized nasal cannulaThis variation in the percentage of gas delivered to the same sizecannula is due to the variation in the total cross-sectional area of thenares of both users. For the same size nasal cannula, a user with largernares will deliver a smaller percentage of the total exhaled gas to theports of the nasal cannula than a user with smaller nares. Tlus, aconventional nasal cannula cannot accurately measure the airflow for aplurality of users having different sized nares.

In addition to detecting and measuring quantities associated with therate of volume of air being delivered to a patient, there are also manyinstances where it is important to detect other characteristicsassociated with respiration, such as snoring. The onset of snoringandlor the intensity of snoring can be used, for example, as a triggerto initiate or control the level of a positive pressure therapy providedthe patient. Also, the presence, intensity and/or duration of snoringcan be used as a diagnostic tool in determining whether the patientsuffers from a sleep and/or breathing disorder.

It is known to use a microphone or pressure sensor mounted on theexterior of the patient's neck to detect sounds or throat vibrationsgenerated by the snore. In many situations, these sensors are mounted onthe user as an individual unit and are not connected to other structuresworn by the patient. This can result in incorrect or inefficientplacement of such sensors. Also, conventional snore sensing devices arequite susceptible to noise. For example, microphones can pick upexternal sounds not produced by the patient, such as snoring of a personor animal near the patient, and/or sounds not resulting from snoring,such as coughing. Pressure sensors can be adversely effected by bodymovements, such as normal movements that take place during the nightand/or throat vibrations resulting from coughing.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide apatient monitoring device for monitoring and/or quantitatively measuringa physiological characteristic of the patient, and, in particular, acharacteristic associated with respiration, that does not suffer fromthe disadvantages of convention airflow/volume meters and snoredetectors. This object is achieved by providing a user interface havingan interior portion that communicates with an airway of a user such thatsubstantially all gas inhaled and exhaled by the user enters theinterior portion of the user interface. At least one vent element isassociated with the user interface and connects the interior portion ofthe user interface with the ambient atmosphere outside the userinterface. The vent element and the user interface define a flow elementacross which a pressure differential is created during inspiration andexpiration. This pressure differential is a pressure difference betweena first pressure within the interior portion of the user interface andthe pressure of the ambient atmosphere outside the user interface. Asensor coupled to the interior portion of the user interface measures afluid characteristic resulting from the pressure differential andoutputs a signal indicative of that fluid characteristic. This signalcan be used to monitor and/or measure physiological characteristics ofthe patient. In a preferred embodiment of the present invention, thesignal output by the sensor corresponds to a characteristic associatedwith respiration and a processing unit receives this signal anddetermines a quantitative value for the characteristic associated withrespiration based thereon.

It is yet another object of the present invention to provide a patientmonitoring method for monitoring and/or quantitatively measuring aphysiological characteristic of the patient that does not suffer fromthe disadvantages of conventional patient monitoring methods. Thisobject is achieved by providing a method that includes the steps ofproviding a user interface having an interior portion adapted tocommunicate with an airway of a user such that substantially all gasinhaled and exhaled by the user enters the interior portion of the userinterface. The user interface also has at least one vent elementassociated therewith for communicating the interior portion of the userinterface with the ambient atmosphere outside the user interface. Thevent element and the user interface define a flow element across which apressure differential is created during inspiration and expiration. Thispressure differential is the pressure difference between a firstpressure within the interior portion of the user interface and thepressure of the ambient atmosphere outside the user interface. The nextsteps in the method of monitoring and/or quantitatively measuring aphysiological characteristic of the patient include passing a gas acrossthe flow element during inspiration and expiration, measuring a fluidcharacteristic resulting from the pressure differential between thepressure within the interior portion of the user interface and ambientatmosphere, and outputting a signal based on the measured fluidcharacteristic. In a preferred embodiment of the present invention, themethod also includes using the output signal. to determine aquantitative value for the physiological characteristic of the patient.

It is a further object of the present invention to provide a patientmonitoring apparatus and method for detecting an analyzing a patient'ssnore. This object is achieved by providing a patient monitoringapparatus that includes a user interface having an interior portion thatcommunicates with the airway of a user, a device for measuring gas flowbetween the user and the user interface or a pressure within the userinterface created by the gas flow, and a processing unit that determinesa quantitative volume for an amount of gas displaced during at least aportion the user's snore based on a signal output by the measuringdevice. In a futher embodiment of the present invention, the processingunit determines a location of a structure in the user that causes thesnore based on this quantitative volume.

These and other objects, features and characteristics of the presentinvention, as well as the methods of operation and functions of therelated elements of structure and the combination of parts and economiesof manufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious figures. It is to be expressly understood, however, that thedrawings are for the purpose of illustration and description only andare not intended as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-2 are schematic diagrams of conventional pneumotach airflowmeters;

FIG. 3 is a schematic diagram of a conventional nasal cannula airflowmeter;

FIGS. 4A and 4B schematically illustrate a first embodiment of firstportion of an interface meter according to the principles of the presentinvention;

FIG. 5 is a schematic diagram a second portion of the interface meterillustrated in FIG. 4;

FIGS. 6, 7 and 8 are charts illustrating the relationship between theoutput from a sensor coupled to an interface device and the actual flowthrough the interface;

FIG. 9 is a more detailed circuit diagram of the interface meterillustrated in FIG. 5;

FIGS. 10A and 10B are waveforms illustrating the flow and volume ofpatient respiration measured using the interface meter according to thefirst embodiment of the present invention;

FIG. 11A is a waveform illustrating the uncalibrated flow signal outputby the sensor portion of the interface meter in the presence of usersnoring, and

FIG. 11B is a waveform illustrating the calibrated (actual) flow signaloutput from the interface meter (inhale only) in the presence ofsnoring;

FIG. 12 is a waveform illustrating a flow signal produced by theinterface meter of the present invention in the presence of snoring thatdemonstrates how the present invention is used to analyze patientsnoring;

FIG. 13 is a schematic diagram of a circuit used to analyze a patient'ssnore according to the principles of the present invention;

FIG. 14 illustrates various configurations for a first embodiment of theinterface meter according to the principles of the present invention;

FIG. 15 illustrates a second embodiment of an interface meter accordingto the principles of the present invention;

FIG. 16 illustrates a third embodiment of an interface meter accordingto the principles of the present invention;

FIG. 17 illustrates a fourth embodiment of an interface meter accordingto the principles of the present invention;

FIG. 18 illustrates a fifth embodiment of an interface meter accordingto the principles of the present invention; and

FIG. 19 illustrates a sixth embodiment of an interface meter accordingto the principles of the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS OF THEINVENTION

FIGS. 4A and 41 schematically illustrate a first embodiment of aninterface meter 70 according to the principles of the present invention.Meter 70 includes a user interface 72 in which in this embodiment is amask worn over the nose and/or mouth of the patient (not shown). Itshould be noted that the terms “user” and “patient” are usedsynonymously through this document. A wall 73 of user interface 72defines an interior portion 74 that receives the user's nose and/ormouth when worn by the patient. As the user breathes into the userinterface, gas is transferred between the user and interior portion 74of user interface 72. A plurality of holes 76 are provided in wall 73 ofuser interface 72 to exhaust exhaled gas firom interior portion 74 tothe ambient atmosphere outside user interface 72. See FIG. 4A.Conversely, gas inhaled by the user enters interior portion 74 of userinterface 72 through holes 76 before being inhaled by the user. See FIG.4B.

A sensor 78 is coupled to a hole 80 in the user interface to measure afluid characteristic, such as a flow rate or a pressure differential,associated with the transfer of gas between interior portion 74 of userinterface 72 and ambient atmosphere. In the embodiment illustrated inFIGS. 4A and 4B, sensor 78 is coupled to user interface 72 such that aportion of the gas entering or exiting interior portion 74 of userinterface 72 passes through the sensor. The size and shape of sensor 78,hole 80 and a tubing 82 connecting sensor 78 to hole 80 are selected soas to minimize the resistance to flow between interior portion 74 andthe area outside the mask imposed by sensor 78, hole 80 and tubing 82.In the illustrated embodiment, sensor 78 is an air flow meter thatmeasures the rate of flow of gas passing through the meter.

Holes 76 provided in user interface 72 function in much the same manneras the flow element in a conventional pneumotach. Namely, holes 76create a slight resistance to the flow of gas into or out of interiorportion 74 of user interface 72 so that during inhalation andexhalation, a pressure differential is created between interior portion74 of user interface 72 and the ambient pressure outside the mask. Thispressure differential causes gas to flow through the circuit defined bytubing 82 and sensor 78 so that the rate of flow of gas through sensor78 can be qualitatively measured by sensor 78.

For an incompressible fluid or gas, the flow of a fluid into an areamust equal the flow of the fluid out of that area (Q_(IN) =Q_(OUT)). Itshould be noted that the terms “fluid” and “gas” are usedinterchangeably throughout this document. Applying this principle tointerface 72, establishes that the flow of fluid into interior portion74 from the user during exhalation Q_(TOT IN) must equal the flow offluid Q₁, Q₂, . . . Qn from the mask out holes 76, assuming that thereare no unaccounted for leaks in the mask or at the user/mask interface.See FIG. 4A. Similarly, the flow to the user during inhalationQ_(TOT OUT) must equal the flow into the mask through holes Q₁, Q₂, . .. Qn, again, assuming that there are no unintentional leaks in the maskor at the user/mask interface. See FIG. 4B. Thus, Q_(TOT)=Q₁+Q₂+. . .Qn.

While the illustrated embodiment describes the mask interface as havinga plurality of holes defined directly in the wall of the interface, itis to be understood that the present invention is not limited to thisparticular configuration for communicating the interior portion of theinterface to the ambient atmosphere. On the contrary, the presentinvention contemplates that any venting structures that communicates theinterior portion of the interface to the ambient atmosphere, whilecreating a sufficient pressure differential can be used. For example,venting can be achieved in a mask that has no exhaust holes by attachingan adapter tube to the inlet/outlet port in the mask. Holes can beprovided in the adapter tube that communicate the interior portion ofthe adapter tube, and hence the interior portion of the mask, to ambientatmosphere. The combined mask and adapter is equivalent to userinterface 72 illustrated in FIGS. 4A and 4B. It can also be appreciatedthat the venting structures need not be provided directly in the mask.Also, the venting mechanism, such as holes 16, can have any shape,pattern, or number of holes so long as they function for their intendedpurpose—to communicate the interior of the user interface to ambientatmosphere while creating a sufficient pressure differential to producea fluid characteristic that can be measured by sensor 78. Also, theventing mechanism need not be defined by fixed diameter holes. On thecontrary, the diameter or degree of opening of the venting structure canvary.

In the illustrated embodiment of the present invention, the area of hole80 is fixed relative to the total area of the remaining holes 76 in userinterface 72, so that the flow of gas Q₅ out of the mask through sensor78 is a known fraction of the total flow of gas out of interior portion74 of user interface 72 during expiration. Conversely, the flow of gasQ₅ into the mask through sensor 78 is a known fraction of the total flowof gas into interior portion 74 during inspiration. Sensor 78 measuresthe flow of gas Q₅ passing therethrough in either direction and outputsa signal 84 indicative of that flow and of the direction of the flowthrough the sensor. The rate of flow through the sensor is acharacteristic of the gas passing through the mask interface and, asnoted above, results from the pressure differential created by the flowelement, which in this embodiment is defined by providing holes directlyin the mask.

Because the portion of gas passing through sensor 78 is a known fractionof the total amount of gas passing through holes 76 and 80, the totalflow of gas to and from the interior portion of user interface 72 can bedetermined from the measured flow through meter 78. Ideally, themeasured flow through sensor 78 is linearly related to the total flowQ_(TOT) into or out of interior portion 74 of user interface 72, so thatonce the flow through sensor 78 is known, the total flow into or out ofthe mask can be readily determined by applying a multiplying factor tosignal 84 output from sensor 78. This can be accomplished, for example,by amplifying signal 84 by a predetermined amount.

It has been determined, however, that the flow measured by sensor 78 istypically not linearly related to the total flow through the userinterface. This is so because the relationship between the total flowQ_(TOT) through interior portion 74 of user interface 72 and themeasured flow through sensor 78 is dependent upon a number of factors,such as the number and size of holes 76, the shape of interface 72, thedistance of the sensor sampling port from the pressure source, theresistance to flow through the sensor and associated components, and thelocation of hole 80 in the mask to which the sensor is attached. Thus,additional processing typically must be performed on signal 84 beforethat signal accurately indicates the actual total flow through the userinterface.

Regardless of whether the relationship between the flow through thesensor and the total flow through the mask is linear or non-linear, aslong as the structure of the interface meter does not change, thedetermination of the total flow Q_(TOT) into interior portion 74 of userinterface 72 using the measured flow through sensor 78 will besubstantially the same for all users regardless of the physicalcharacteristics of the patient wearing the interface meter. Thus, oncesensor 78 is calibrated for a particular interface 72 with fixedstructural features, i.e., once the relationship between the output ofsensor 78 to the total flow through the mask interface is established,the same interface meter 70 can be used on a wide variety of patients tomeasure characteristics associated with respiration quantitatively, suchas the flow and/or volume of gas provided to the patient.

In a preferred embodiment of the present invention, sensor 78 is a massairflow sensor, such as the AWM2100V sensor manufactured by HoneywellInc., which outputs a range of analog voltages corresponding to apredetermined range of airflow rates through the sensor. The output fromthe AWM2100V is a positive and negative differential signal thatcorresponds to the rate and direction of flow through the sensor. TheAWM2100V sensor is particularly well suited for use in measuring theamount of gas passing through a portion of user interface 72 because theAWM2100V is capable of accurately measuring a very small flow. Forexample, it has been determined that the pressure drop needed togenerate flow across the AWM2100V at full scale is only 0.5 cm H₂O.Because the flow of breathing gas through sensor 78 can be quite small,the pressure drop across user interface 72 needed to create a flowthrough the AWM2100V is also quite small. As a result, user interface 72can have an extremely low resistance so that gas flows relatively easilyinto and out of interior portion 74. Decreasing the flow resistance,which is accomplished by reducing the pressure drop across the userinterface, i.e., across the flow element defined by the mask and theholes in the mask, is achieved, for example, by providing more holes inthe mask andlor increasing the size of the holes so that breathing gasflows more freely between interior portion 74 and the area outside userinterface 72.

One advantage achieved by making the mask resistance as low as possibleis to provide a good (leak free) mask seal with the patient. The lowerthe mask resistance, the more likely there will be no leaks at themask/patient interface. Unintentional leaks in the mask or in themask/patient interface can be taken into consideration in determiningthe total flow to and from the patient based on the measured flowthrough sensor 78. For example, the leak estimation algorithms taught byU.S. Pat. Nos. 5,148,802; 5,239,995; 5,313,937; 5,433,193; and5,632,269, the contents of which are incorporated herein by reference,can be used to determine unintentional leaks in the mask or mask/patientinterface. If these unintentional leaks are minimized to aninsubstantial amount, the use of leak estimation and correctiontechniques can be avoided.

It has been determined that a good seal is achieved as long as thepressure within interior portion 74 of user interface 72 is between −2cm H₂O to 2 cm H₂O. The relatively low flow resistance through theAWM2100V allows the pressure within the mask to be within this range.Thus, the assumption that there are no mask leaks other than throughholes 76 and 80 is valid. For example, it has been determined that thepressure in user interface 72, even with the pressure drop caused bytubing 82 and a bacteria filter (not shown) placed between userinterface 72 and sensor 78, is 1 cm H₂O at a flow rate of 100 liters perminute (1pm).

In any event, even if the pressure drop needed to generate flow acrossthe sensor exceeds 2 cm H₂O, unintentional leaks at the mask/patientinterface can be eliminated by increasing the sealing force applied onthe mask to hold the mask on the patient and/or by providing an improvedseal between the mask and user, such as an adhesive seal or a largersealing area.

The present invention also contemplates that sensors other than anairflow sensor can be use as sensor 78. For example, sensor 78 can be apressure sensor. An example of a suitable pressure sensor in adifferential pressure sensor that directly measures the differentialbetween interior portion 74 of user interface 72 and a pressure of theambient atmosphere outside the user interface. This pressuredifferential, like the flow of gas through the sensor in the previousembodiment, is due to the restriction in flow between ambient atmosphereand the interior of the mask created by the flow element, which in thisembodiment is defined by the holes provided in user interface 72.Another suitable sensor is an absolute pressure sensor that measures thepressure of interior portion 74 relative to a fixed reference pressure.Any sensor, such as an airflow, pressure or quantitative temperaturesensor, that is capable of measuring a fluid characteristic created bythe pressure differential caused by the flow element and that is capableof outputting a signal indicative of that characteristic can be used assensor 78.

If sensor 78 is a pressure sensor, gas does not pass through the sensor.Instead, for a differential pressure sensor, one side of a diaphragm insensor 78 communicates with interior portion 74 of user interface 72 andthe other side of the diaphragm communicates with ambient atmosphere.The pressure sensor measures the pressure differential ΔP between afirst pressure within the interior portion of the user interface and theambient atmospheric pressure outside the user interface. Once thispressure differential is known, the actual flow rate Q_(IN) of gaspassing through the mask interface can be determined, for example, usinga look-up table based on the known relationship between pressure andflow, i.e., ΔP=RQ². A similar approach is used of the pressure sensor isan absolute pressure sensor.

Regardless of whether sensor 78 is an airflow sensor, a pressure sensor,or any other type of sensor, the signal output by the sensor istypically an analog signal. If sensor 78 is an airflow sensor, signal 84corresponds to the rate of flow of gas through the sensor and indicatesthe relative direction of flow. If sensor 78 is a differential pressuresensor, signal 84 corresponds to a pressure differential across the flowelement and also indicates a relative direction of flow based on whetherthe pressure within the interface is greater or less than ambientpressure. It can be appreciated that signal 84 corresponds to acharacteristic associated with respiration because signal 84 can be usedto quantitatively determine a characteristic associated withrespiration, such as patient flow or volume. Even in its raw,uncalibrated form, signal 84 can be used to differentiate betweeninspiration and expiration and/or to detect snore, and thus, correspondsto the respiratory characteristic of inspiration, expiration and/orsnore. The value of analog signal 84 represents a valve of one of thesecharacteristics of respiration. Examples, of characteristics associatedwith respiration that can be determined using signal 84 are discussed ingreater detail below.

As shown in FIG. 5, signal 84 from sensor 78 is provided to an amplifier86 and the output of amplifier 86 is provided to an analog-to-digital(A/D) converter 88. The digital output 90 of A/D converter 88 isprovided to a processor 92 that corrects for the non-linearity in theoutput of sensor 78 so that the signal 94 output from processor 92 is adigital signal indicative of a quantitative value for a characteristicassociated with respiration.

For example, in one embodiment of the present invention discussed above,signal 84 from sensor 78 is a signal indicative of the rate of flow ofgas through the sensor. However, as discussed above, this signal istypically not linearly related to the total rate of flow of gas Q_(TOT)to or from the interior portion 74 of the user interface. To correct forthis non-linearity, signal 84 is provided to processor 92. Processor 92determines the quantitative (actual) value for the total flow of gasQ_(TOT) entering or exiting the interior portion 74 of user interface 72based on signal 84. The details as to how this is accomplished arediscussed below. It is to be understood, that, based on signal 84,processor 92 can determine characteristics associated with respirationother than flow rate. For example, by integrating the corrected flowsignal, processor 92 can output a signal representing the total volumeof gas V_(TOT) exiting or entering the interior portion of the userinterface.

In the illustrated embodiment, a digital-to-analog converter 96 convertssignal 94 from processor 92 into an analog signal 98 and provides analogsignal 98 to an output and/or storage device 100. In a preferredembodiment of the present invention, output device 100 is a monitor oran LED display, that converts signal 98 into a human perceivable outputindicative of the characteristic associated with respiration, such asrate or volume of flow to and from the user. It is also preferable tostore signal 98 in a memory for use in evaluating the respiratoryconditions of the patient. Alternatively or in addition to the aboveembodiment, output 94 from processor 92 can be provided in its digitalformat to a digital output device 99, such as a digital display, memory,terminal, and/or communication system.

As noted above, because the output from sensor 78 is typically notlinearly related to the rate of flow through the mask interface over theentire range of airflows to and from the patient, processor 92 mustcorrect for the non-linearity in the output of sensor 78. In a preferredembodiment of the present invention, processor 92 calculates the totalflow QTro entering or exiting interface 72 based on the output fromsensor 78 using a lookup table, which is determined from apreestablished relationship between the output from sensor 78 and theflow through the interface. FIG. 6 is a graphical representation of thisrelationship. It should be understood that the graph in FIG. 6 isdetermined for a specific mask interface. The relationships establishedby the curve in FIG. 6 do not apply to all interfaces. Thus, for eachdifferent type of interface to which the processor is to be used, therelationship between the output of the sensor and the actual,quantitative value for the respiratory characteristic of interest mustbe determined beforehand so that this relationship can then be used todetermine the quantitative value for the desired respiratorycharacteristic.

Curve 102 in FIG. 6 illustrates the relationship between the signaloutput by sensor 78 for a first type of mask interface and the flowthrough that interface. The vertical axis of the graph in FIG. 6corresponds to the output of sensor 78, which is typically in a range of−60 mV to +60 mV for the AWM2100V sensor. The horizontal axis representsthe total flow Q_(TOT) into or out of the interface. The portion ofcurve 102 to the right of the zero flow mark on the horizontal axisrepresents flow in a first direction through the sensor, for exampleexpiration, and the portion of curve 102 to the left of the zero flowpoint represents flow in a second direction, opposite the firstdirection, for example inspiration.

It can be appreciated from FIG. 6 that for the particular sensor andtype of interface used to generate curve 102, the output from sensor 78is not linearly related to the rate of flow through the mask interface.This is particularly true near the zero flow rate. However, by knowingthe relationship between the output of sensor 78 and the total flow, theactual, quantitative flow through the mask can be readily determined.

It can be further appreciated that curve 102 will have different shapesdepending on the type of sensor and interface being used. However, oncethe relationship between the sensor output and the flow through theinterface is determined, this relationship remains valid independent ofthe physical characteristics of the patient using the interface meter.Thus, unlike nasal cannulas, the same interface meter can be used toquantitatively determine a characteristic associated with respiration,such as the flow rate, for a wide variety of users.

FIG. 7 is similar to FIG. 6 in that it is a graph illustrating therelationship between the signal output by sensor 78 for a particular typof mask interface and the flow through that interface. However, thevertical axis in FIG. 7 denotes a linearly amplified output of sensor78, which corresponds to signal 122 in FIG. 5. The signal output fromsensor 78 illustrated in FIG. 7 has been amplified so that the voltagerange of the signal is between −5 V and +5 V. FIG. 7 includes a firstcurve 104, illustrated by a solid line, that represents thevoltage-total flow relationship for flow through sensor 78 in a firstdirection (typically during exhalation) and a second curve 106,illustrated by a dotted line, that represents the voltage-total flowrelationship for flow through sensor 78 in a second direction (typicallyduring inhalation) opposite the first direction. In the illustratedembodiment, the output from sensor 78 is positive during expiration andnegative during inspiration. It is to be understood, however, that thisrelationship could be reversed

In FIG. 7, curves 104 and 106 representing the flow during expirationand inspiration, respectively, are superimposed on one another todemonstrate that the voltage-flow characteristics are substantially thesame regardless of the direction of flow through sensor 78. Thus, thesame relationship between the sensor output and the flow through theinterface can be used regardless of the direction of flow through theinterface, i.e., during inspiration and expiration, thereby simplifyingthe determination of the flow through the interface based on themeasured output of sensor 78. It is possible, however, to use separaterelationships to determine a quantitative value for a characteristicassociated with inspiration and a characteristic associated withexpiration.

In a preferred embodiment of the present invention, the knownrelationship between the output of sensor 78 and the flow through themask interface, as illustrated by the curves in FIGS. 6 and 7 forexample, is used to generate a lookup table. This table is used todetermine the actual flow through the mask interface from the output ofsensor 78. However, the present invention contemplates that techniquesother than a look-up can be used to determine a quantitative measure ofa characteristic associated with respiration from the raw signal outputfrom sensor 78. For example, once the voltage-total flow relationshipfor the interface meter is established, the flow can be calculated froman equation defining this relationship. For example, curves 104 and 106in FIG. 7 can be generally defmed by the following third orderpolynomial equation:

y=−2.208×10⁻⁶x³+5.982×10⁴x²−2.731×10⁻³x+9.165×10⁻³,

where y is the linearly amplified output of sensor 78 and x is the flowinto or out of the mask interface. Once y is determined by sensor 78,processor 92 can solve for x to determine the total flow into or out ofthe interface. As noted above, separate equations or lookup tables canused to determine the flow through the mask during expiration andexpiration, thereby improving the accuracy of the output of theinterface meter if the relationship between the output of sensor 78 andflow through the interface is not the same for flow in both directionsthrough the sensor.

It should be understood that the above equation and the graphillustrated in FIGS. 6 and 7 defining the relationship between theoutput of sensor 78 and the total flow into or out of the mask interfaceapply only to a particular type of interface having a predeterminedstructure. If, for example, more holes are added or the mask shape orsize is altered, the relationship between the output of sensor 78 andthe total flow into or out of the mask interface may change, requiringrecalibration of processor 92 so that a different curve is used todetermine the quantitative value for the desired respiratorycharacteristic based on the output of the sensor.

For example, FIG. 8 illustrates three curves 101, 103 and 105 definingthe relationship between the pressure measured by sensor 78 and the flowthrough the interface for three masks having different structuralcharacteristics. Curve 101 associated with a first mask is nearly astraight line, meaning that there is nearly a linear relationshipbetween the pressure measured by sensor 78 and the total flow throughthe interface. FIG. 8 also demonstrates that if sensor 78 is a pressuremonitor, the same techniques used to generate the total flow through themask, i.e., using a look-up table or equation derived from therelationships illustrated in FIG. 8, can be used to determine thequantitative value for characteristic associated with respiration, suchas flow through the interface.

So long as a batch of mask interfaces are manufactured with the samestructural characteristics, the same calibration, i.e., voltage—flowcurve, can be applied to all of the mask interfaces in that batch. Byproviding each processor with the same voltage-total flow relationshipthere is no need to calibrate each interface meter individually. Inshort, the interface meters of the present invention can be commonlycalibrated so long as they share the same structural characteristics forthe interface. The operating characteristics of the interface meter donot vary with the physical characteristics of the user, as is the casewith conventional nasal cannula flow meters.

It is to be further understood that processor 92 can contain a number ofdifferent lookup tables and/or equations associated with a variety ofdifferent interface devices so that the same processor can be used inconjunction with a number of different types or configurations ofpatient interfaces, so long as the proper lookup table or voltage-totalflow equation is used with the selected interface. In this embodiment, aselector is provided so that the user can select the type of interfacebeing used with the interface meter. Processor 92 then uses the correctlookup table or equation or other technique for determining thequantitative value for a patient's physiological characteristic based onthe selected interface. For example, a memory portion in processor 92can contain three lookup tables associated with three different masksizes. The user selects the mask size being used and inputs thisselection to processor 92. Processor 92 then uses the correct lookuptable for the selected mask size to determine the quantitative value forthe flow through the mask interface based upon the output from sensor78.

As discussed above, a primary function of processor 92 in the presentinvention is to convert the signal from sensor 78 into a signal thataccurately represents the flow of breathing gas into or out of the userinterface. This is necessary because it is believed to be difficult tosituate the various structural elements of the interface and sensor suchthat the signal output from sensor 78 is linearly related to the flowthrough the mask to which the sensor is attached. If, however, asuitable configuration can be established, the linearizing functionperformed by the processor will not be necessary. Instead, processor 92will merely provide a multiplying function to calculate the total amountof breathing gas passing through the mask interface from the knownfraction of breathing gas passing through sensor 78. Alternatively,processor 92 can be eliminated and the multiplying function can beperformed using circuitry, for example, by adjusting the gain inamplifier 86 of FIG. 5.

FIG. 9 is a more detailed diagram of the circuit schematicallyillustrated in FIG. 5. Gas passes through sensor 78 in a firstdirection, as indicated by arrow 107, during expiration and in a seconddirection opposite the first direction, as indicated by arrow 108,during inspiration. Amplifier 110 sets the control for a heater that isused in the Honeywell airflow sensor to measure the flow ratetherethrough. Outputs 112 and 114 of sensor 78 are positive and negativedifferential signals representing the flow measured by sensor 78 and areprovided to a pair of amplifiers 1 16 and 118, respectively. Outputs ofamplifiers 116 and 118 are provided to a differential amplifier 120.Amplifiers 116, 118 and 120 define amplifier 86 in FIG. 5 and convertthe dual outputs of sensor 78 into a single analog signal 122. In apreferred embodiment of the present invention, amplifiers 110, 116, 118and 120 are provided on a same integrated circuit, such as the LMC660CNQuad OP-AMP manufactured by National Semiconductor.

Signal 122 from amplifier 86, which is referred to as a raw oruncalibrated signal because it typically does not linearly correspond tothe respiratory characteristic of interest, is provided to A/D converter88, such as an ADC10831 converter manufactured by NationalSemiconductor. Digital output 90 of A/D converter 88 is provided toprocessor 92. In the illustrated embodiment, processor 92 is thePIC16C84 manufactured by Microchip Inc. Processor 92 operates at a clockspeed set by oscillator 124 to calculate the flow Q_(TOT) entering orexiting interface 72, for exanple, based on the output from sensor 78 asdiscussed above. It is to be understood, that any combination of thecircuit components illustrated in FIG. 9 can be provided on a singlechip. For example, A/D converter 88, processor 92, and D/A converter canbe fabricated on the same chip for ease of manufacturing the interfacemeter of the present invention.

In one embodiment of the present invention, processor 92 uses a lookuptable or a voltage-total flow equation established for a particular typeof interface 72 to determine the flow Q_(TOT) entering or exiting theinterface based on signal 90 from A/D converter 88. In the illustratedembodiment, output 94 of processor 92 is a signal indicative of the flowentering or exiting the interface and is provided to D/A converter 96where it is converted into a pair of analog signals 98, which arepositive and negative signals, respectively, depending on the directionof flow through sensor 78. In the illustrated embodiment D/A converter96 is a DAC0854 converter manufactured by National Semiconductor. Afirst pair of variable resistors 126 set the positive gain for theanalog output of D/A converter 96 and a second pair of variableresistors 128 set negative gain. Analog signals 98 are provided to adisplay 100, such as an LCD or LED display, where they are convertedinto an output that is capable of being perceived by humans.

In the illustrated embodiment, analog signals 98 are also provided to apair of output terminals 130 and 132 so that signals 98, which representthe actual (quantitative) flow of breathing gas passing through theinterface, can be provided to external components, such as a display,data storage device, alarm system, printer, additional processingelements, and/or data communication system, such as a modem. It is to beunderstood, however, that any of these components could be providedwithin the circuitry illustrated in FIG. 9 on the same card or circuitboard.

FIG. 10A illustrates a waveform 134 of the flow through sensor 78 duringinspiration and expiration in liters per minute (1pm) generated by acomputer using signal 98 taken at terminals 130 and 132 in FIG. 9. FIG.10A is one example of how the signal produced by processor 92 could beoutput in human perceivable format. FIG. 10B illustrates a waveform 136of the tidal volume for the same flow rate of breathing gas passingthrough sensor 78 in liters, which is also generated by a computer usingsignal 98 taken at terminals 130 and 132. Waveform 136 in FIG. 10B canbe generated, for example, by integrating the flow signal 134illustrated in FIG. 10A. The smoothness of waveforms 134 and 136illustrated in Figs. 10A and 10B can be improved by increasing theprocessing speed of processor 92. This could be accomplished, forexample, by increasing the oscillating frequency of oscillator 124 inFIG. 9.

FIG. 11A illustrates a waveform 138 that corresponds to the uncalibratedanalog flow signal 122 output from amplifier 86 in FIG. 8 duringinhalation and exhalation. The points in FIG. 11A where waveform 138crosses the X axis correspond to points where the patient's breathingswitches from inspiration to expiration or from expiration toinspiration. Thus, these points can be used as trigger points orreference points for the application of a respiratory therapy, such asan application of positive pressure to the airway or an application ofelectrical stimulation to the muscles in the patient.

Waveform 138 was generated while the user was asleep and snoring. Therapid signal fluctuations 137 at each apex of inhalation in waveform 138correspond to the rapid flow variations that take place in the user'srespiratory system during snoring. One embodiment of the presentinvention detects these rapid fluctuations in the raw signal 122 outputfrom the sensor to determine the onset, intensity and duration ofsnoring. This can be accomplished in a variety of ways, for example, bycomparing the rate of change in signal 138 to predetermined thresholds.Because this rapid variation in flow (snore) can be easily detected fromwaveform 138, the signal from sensor 78, even if not corrected byprocessor 92, can be used, for example, as a trigger for a therapyintended to relieve such snoring or as a reference point from whichtherapy is to begin.

FIG. 11B illustrates a waveform 140 that corresponds to the signaloutput from processor 92 based on the signal illustrated in FIG. 11A. Inother words, waveform 140 corresponds to the quantitative signalproduced by processor 92 based on the raw signal illustrated in FIG.11A. It should be noted that FIG. 11B illustrates only the inspirationportion of the patient's flow, which is the equivalent of the output atone of terminals 130 and 132 in FIG. 9. As with wavefonn 138 in FIG.11A, waveform 140 in FIG. 11B exhibits relatively large and rapidfluctuations 139 during inspiration due to the patient's snoring. Theserapid fluctuations can be detected in a variety of fashions, forexample, by using a threshold detector, to signal the onset of snoring.It can be appreciated if the processing speed of processor 92 isincreased, the rapid fluctuations in the apex of waveform 140 would beeven more well defined. In fact, the sensitivity of the presentinvention is so great that the gas displaced by each individual snorevibration can be determined.

It can be further appreciated that the present invention can determine awide variety of information based on the output from sensor 78. Forexample, as noted above, by integrating the quantitative value for flow,which can be done either by processor 92 or using addition componentsthat are either internal or external to the circuit illustrated in FIGS.5 and 9, the interface meter of the present invention also calculatesthe volume V_(TOT) of breathing gas entering or exiting the interface.Calculating the volume V_(TOT) can be done in place of or in addition todetermining the flow Q_(TOT) of breathing gas passing through theinterface. The present invention contemplates providing additionaldigital-to-analog converters similar to D/A converter 96, additionaloutput devices similar to output device 100, as well as additionaloutput terminals similar to terminals 130 and 132 so that any additionalinformation, such as volume V_(TOT), can be calculated and provided tothe user, a third party, or to a data output and/or storage medium.

Knowing the quantitative value for the patient flow makes it possible todetermine a number of physical characteristics associated withrespiration. This can be done using processor 92 or other circuitrybased on the signal output from processor 92 and/or, where possible, theraw signal output from sensor 78. For example, the present inventioncontemplates using either the raw output of sensor 78 or the flow signaloutput from processor 92, such as that illustrated in FIG. 10A, todetermine the patient's breathing rate, typically in breaths per minute(bpm), minute ventilation, peak expiratory flow, inhalation time,exhalation time, and inhalation to exhalation (I:E) ratio. Also, thepresent invention contemplates using the volume signal, such as thatillustrated in FIG. 10B, to determine the patient's exhalation volumeand inhalation volume.

In addition to determining a number of physical characteristics, thepatient flow, which is characterized by the raw signal from sensor 78(FIG. 11A) or the quantitative signal from processor 92 (FIG. 10A), canbe used for a variety of purposes. For example, as noted above, thepresence, frequency, duration or intensity of rapid fluctuationsindicative of snoring can be used to trigger the application of atherapy, such as an airway pneumatic pressure support, to relieve thesnoring. The detection of snoring using the patient's flow signal (rawor quantitative) can be used to auto-titrate a pressure support device.Auto-titration is accomplished, for example, by increasing the pressureprovided by a pressure support device if the presence or intensity ofsnoring, or more generally, the presence of any event indicative of theonset of an airway obstruction, is detected, and by decreasing thepressure if such events are not detected for a predetermined period oftime. This same principle can be employed with other devices, such as anelectrical stimulation device, that is used to relieve the obstruction.Auto-titration can also be accomplished based on the rise time of theflow signal. An increase in rise time can indicate an increase in airwayresistance, and hence, the onset of an airway obstruction. This increasein rise time can be detected by the present invention and used toincrease the pressure support provided to the patient. The oppositeprocess can be carried out if a decrease in the flow signal rise time isdetected.

It is also possible to determine specific characteristics of a patient'ssnore based on the signal output from sensor 78. For example, thefrequency of the snore can be determined from the rapid fluctuations inthe flow signal, either from the raw signal output from the sensor orthe calibrated, quantitative signal derived from the raw signal. It isknown that the frequency of the snore signal can indicate the physicallocation of the structure or structures in the patient causing thesnore. See, for example, the article by S. J. Quinn et al. entitled,“The Differentiation of Snoring Mechanisms Using Sound Analysis,” pages119-123 of Clinical Otolarnynhology, Vol. 21, 1996. Knowing the locationof the tissue that is causing the snore is important in determining howto best treat the snore.

As noted above, the sensitivity of the interface meter of the presentinvention is great enough that it can detect the amount of gas displacedby each individual snore vibration. For example, FIG. 12 illustrates aflow signal 151 generated by the interface meter of the presentinvention in the presence of patient snore 153. Snoring 153 appears inflow signal 151 as a series of high frequency oscillations 155 in flowsignal 151 that oscillate about a central axis 157. Each oscillationdisplaces an amount of gas corresponding to the area 159 defined by axis157 and the curve defining the oscillation.

As noted above, the frequency of a snore can be used to determine thelocation of the structure or structures in the patient that cause thesnore. In a similar manner, the amount of gas displaced by eachindividual snore vibration can also be used to determine the location ofthe snore. The amount of gas displaced by each snore vibration isrelated to the frequency of that snore vibration. For example, the lowerthe frequency of the snore, the more gas will be displaced by eachindividual snore vibration. Therefore, by knowing the amount of gasdisplaced by the individual snore vibrations, the present invention candetermine the location of the structure in the patient that is causingthe snore. Furthermore, because the present invention accomplishes thisfunction based on the amount of gas displaced by each snore vibration,rather than based on the sound produced by the snore, it is moreaccurate and less prone to noise than conventional frequency analysistechniques.

In addition to determining the volume of gas displaced by each vibrationin a patient's snore, the present invention also quantitativelydetermines the volume of the patient's entire snore signal.Quantitatively determining the volume of gas displaced by the patientsnore can be accomplished, for example, as shown in FIG. 13. The output141 of sensor 78 is provided to a low pass filter (LPF) 142 that removesthe relatively high frequency snore from the flow signal so that output143 of low pass filter 142 corresponds to the patient flow without anysnore. The flow signals 141 and 143 output from sensor 78 and LPF 142,respectively, are provided to a subtractor circuit 144 so that theoutput 145 thereof is the raw, uncalibrated analog snore flow signal.Snore flow signal 145 is provided to a processor 146, which uses alook-up table or other technique, to determine the quantitative value ofthe snore flow 147. Integrating only the positive portion of the snoreflow signal 147 in integrator 148 provides a volume accurate snoresignal 149, which can be used to analyze the patient's snore. It is tobe understood, that only the negative portion of the snore flow signalcan be integrated and the same result achieved.

It is to be further understood that other techniques for determining avolume accurate snore signal are contemplated by the present invention.For example, the positive portion of analog signal 145 can be integratedand then software can be used to determine the derivative, which is thenconverted into a quantitative flow signal to determine a quantitativesnore flow signal. This quantitative snore flow signal can then beintegrated to provide the volume accurate snore signal. Also, thedetermination of patient flow, either raw or quantitative, can be madeusing a conventional flow measuring device.

The information generated by the interface meter of the presentinvention can also be used in conjunction with other information aboutthe patient's physiology to determine other characteristics of thepatient. For example, if a capnometer is used to measure the patient'sexpired CO₂, the flow signal and the capnometer information can be usedto determine the volume of CO₂ expired by the patient. The volume of CO₂expelled from a patient during exhalation can be determined from thefollowing equation:${V_{{CO}_{2}} = {{V_{MIX}\left\lbrack \frac{{PCO}_{2}}{P_{MIX}} \right\rbrack}{t}}},$

where V_(MIX) is the volume of gas expired by the patient, PCO₂ is thepressure of carbon dioxide in the gas expired by the patient, andP_(MIX) is the pressure of the gas expired by the patient. As discussedabove, V_(MIX) can be quantitatively determined by the presentinvention. PCO₂ is determined using a capnometer, and P_(MIX) isdetermined using a conventional barometer.

Similarly, the volume of CO₂ expelled from a patient during exhalationcan be determined based on the quantitative flow signal using thefollowing equation:$V_{{CO}_{2}} = {\int_{t_{1}}^{t_{2}}{\left\lbrack {Q_{Patient}\left( \frac{{PCO}_{2}}{P_{MIX}} \right)} \right\rbrack \quad {t}}}$

where: t₂−t₁=inhalation period and Q_(patient) is the flow of gas fromthe patient. This same principle can be used to the measure the volumeof other elements expelled by the patient, such as nitrogen, O₂, CO,water vapor and any other trace elements that can be detected.

Furthermore, the quantitative flow signal output by the presentinvention, in combination with other sensing devices, can be used todetermine a patient's effective minute ventilation, effective tidalvolume, airway dead space, and alveolar volume using conventionaltechniques. If the patient's arterial PCO₂ is also known, furtherinformation, such as the physiologic V_(D)/V_(T), physiologic deadspace, and alveolar dead space can also be determined using conventionaltechniques.

While the items discussed above describe physiological parameters thatare capable of being measured using the present invention, either aloneor in combination with other measuring devices, and processes that canbe performed or controlled based on the information produced by thepresent invention, this list is not intended to be exclusive. On thecontrary, the present invention can be used to determine anycharacteristic about a patient that can be derived from the informationoutput by sensor 78 and/or processor 92. Also, the present invention canbe used in conjunction with any process that is controlled or requiresinformation of the type produced by the present invention, eitherdirectly from the signal output by sensor 78 or processor 92, orindirectly when used combination with other measured physicalcharacteristics.

Although the embodiment of the present invention discussed above hasbeen described for use with a mask-like user interface, it is to beunderstood that a wide variety of user interfaces, which are discussedin greater detail below, can be used in conjunction with the interfacemeter of the present invention. Also, the mask serving as a userinterface in the embodiment illustrated in FIGS. 4A and 4B can have awide variety of configurations. For example, user interface 74 can be anasal mask that covers only the user's nose, a total face mask thatencompasses the user's entire face from chin to forehead, or a helmettype mask that encapsulates the user's head. It should also beunderstood that the term “user interface” is not limited to themask-like structure illustrated in the figures. Quite the contrary, the“user interface” of the present invention can include structures thatattach to the mask-like portion. User interface 72 and tube 82 can bemade from any suitable material. In addition, a bacteria filter can beprovided anywhere along the length of tube 82. It is preferable to use abacterial filter and tubing 82 that have a sufficiently low resistanceso that a suitable amount of gas flows through sensor 78.

FIG. 14 illustrates an. example of a plurality of interface meters 150,152, and 154 according to the first embodiment of the present invention.Each interface meter includes a user interface, which in this embodimentis a mask-type interface, a venting element that communicates theinterior of the interface to ambient atmosphere and a sensor formeasuring a fluid characteristic, such as pressure or flow, resultingfrom the pressure differential between the interior of the mask andambient atmosphere created by the venting element.

Interface meter 150, for example, includes a user interface 158 similarto the interface schematically illustrated in FIGS. 4A and 4B. Theventing element in interface meter 150 is a plurality of holes 160provided in user interface 158. A hollow tube 162 having one endselectively coupled to user interface 158 and a second end selectivelycoupled to a housing 164 communicates the interior of user interface 158with a sensor (such as sensor 78 in the previous figures) in housing164. Housing 164 also contains the circuitry illustrated in FIGS. 5 and9 associated with the sensor. In the illustrated embodiment, a bacteriafilter 166 is provided between the first and second ends of tube 162.

Housing 164 includes a display 167 that corresponds to output device 100in FIGS. 5 and 9 and an on/off activating mechanism 168. Housing 164also includes a selector 170 so that the user can manually select thetype of interface being coupled to housing 164. As discussed above, thisenables the processor to use the appropriate look-up table fordetermining the flow through the interface. Selector 170 and on/offactivating mechanism can be any suitable input device for controllingthe circuitry and/or processing elements of the present invention. Inthe illustrated embodiment, interface meter 150 is AC powered. It is tobe understood, however, that any suitable power supply, such asbatteries, can be used to energize the interface meter.

Interface meter 150 also includes a wireless communication link 169 forcommunicating with a base unit 17. Any suitable wireless communicationsystem, such as an rf communication link or a modem and land linetelephone, cellular, and/or satellite communication system iscontemplated by the present invention.

Interface meter 152 is similar to interface meter 150 except that userinterface 174 in interface meter 152 does not have holes definedtherein. An example of such masks are the nasal mask sold by RESPIRONICSInc. under the trademark “GOLD SEAL”™ and the full face mask that coversthe nose and mouth sold by RESPIRONICS Inc. under the registeredtrademark “SPECTRUM”®. The venting element that communicates theinterior of the interface to ambient atmosphere is an attaching element176 that selectively couples to a hole defined in user interface 174.Attaching element 176 includes a plurality of holes 178 that communicatethe interior of user interface 174 to ambient atmosphere. A headgear 180attaches the user interface to the patient. As with interface meter 150,a hollow tube 162 couples a sensor in housing 164 to the interior ofuser interface 174. Interface meter 152 communicates information withbase unit 172 via a hard wired link 182.

Interface meter 154 includes a first user interface 184 and a seconduser interface 186. Unlike interface meters 150 and 152, the interior ofuser interfaces 184 and 186 communicate directly with a sensor 188 and190, respectively, that is provided on, in or at the user interfaceitself, thereby eliminating the hollow tube of the previous embodiments.Sensor 188 in interface meter 154, like sensor 78 in the previousembodiments, measures a fluid characteristic, such as the flowtherethrough or the absolute pressure within the mask or the pressure inthe mask relative to ambient atmosphere, and outputs a signal via a wire192 to a processor within housing 164. Sensor 190 performs a similarfunction except that there is a wireless communication 194 betweensensor 190 and housing 164. It is to be further understood that thesensor can be provided within the mask.

Base unit 172 processes the information provided by each interface. Forexample, the signal from each interface meter can be the raw flow signalfrom the sensor (sensor 78 in FIG. 5) or the quantitative flow signalfrom the processor (processor 90 in FIG. 5). Base unit 172 can use thesesignals, as discussed above, to determine a variety of respiratorycharacteristics for each patient. Base station 172 can communicate thisinformation, either wirelessly or via wires, to other informationprocessing devices. The illustrated embodiment of the present inventionalso contemplates providing information from base station 172 tovarious-outputlstorage devices, such as a display 196, a printer 198,and a storage device 200.

The multiple interface meter system illustrated in FIG. 14 isparticularly suited for the hospital or sleep lab environment wheremultiple patients are monitored by one caregiver. By employing wirelesscommunications between the components of the interface meter, therespiratory characteristics of a patient can be monitored from a remotelocation, such as the patient's home or while the patient is in transitto a hospital.

While the above embodiment for the interface meter uses a mask-likeinterface that communicates with the airway of the user, the presentinvention is not limited to a mask type interface. Quite the contrary,any interface that communicates with the airway of the user iscontemplated by the present invention. For example, in a secondembodiment of the present invention, as shown in FIG. 15, a pair ofnasal prongs 202 replace user interface 72 of FIGS. 4A and 4B. In allother respects, the second embodiment of the present invention and thefirst embodiment discussed above are the same.

Nasal prongs 202 include protruding portions 204 that insert into thenares of the user. The diameters at each proximal end 206 of protrudingportions 202 are sized to seal the nares into which the protrudingportion are inserted so that gas does not leak around the periphery ofproximal end 206 of protruding portion 204. An opening 208 is defined ata distal end 210 of each protruding portion for communicating aninterior portion of the protruding portion with a nasal cavity of theuser. At least one vent hole 212 is provided in the proximal end ofprotruding portions 204. Vent holes 212 perform the same function asholes 76 in the user interface illustrated in FIGS. 4A and 4B. A sensor(not shown) that performs the same function as sensor 78 in FIGS. 4A and4B is coupled to the interior portion of both protruding portions 204via a hollow tube 214 and short, connecting tubes 216.

FIG. 16 illustrates a third embodiment of an interface meter accordingto the principles of the present invention. The interface meter in thisembodiment includes a incubator chamber 220 as the interface thatcommunicates with the airway of the user. Vent elements 224 are providedin the wall of incubator chamber 200 for communicating the interiorportion of the chamber with ambient atmosphere, in the same manner asholes 76 in user interface 72 of FIGS. 4A and 4B. A sampling port isprovided in the wall of chamber 220 to communicate a sensor 224 with theinterior of the chamber via a hollow tube 226. As with the previousembodiment, tubing 226 can be eliminated and the flow or pressure sensorprovided in direct communication with the interior of chamber 220.Sensor 224 corresponds to the circuitry illustrated in FIG. 5 and 9.

Typically, a breathing gas, such as oxygen or an oxygen mixture, isdelivered to the incubator chamber via a gas supply 222. As a result,there is a constant leak from the chamber through vent elements 224.This leak will offset the raw flow or pressure signal from the sensor,as well as the quantitative flow signal output from the processor, sothat the flow or pressure signal and quantitative flow signal no longervaries about a zero flow or zero pressure axis. Instead, these signalswill fluctuate about a level that corresponds to the leak from thechamber, which corresponds to the flow of breathing gas to the chambervia gas supply 222. In the illustrated embodiment, the processoraccounts for this offset caused by the supply of breathing gas so thatthe output from the processor in sensor 224 is a true representation ofthe patient's inspiration and expiration. This can be done, for example,by subtracting the leak, once determined, from the quantitative signaloutput by the processor. Thus, the present invention outputs aquantitative representation of the flow through the chamber even in thepresence of a constant supply of gas to the chamber.

FIG. 17 illustrates a fourth embodiment of an interface meter 230according to the principles of the present invention. This embodiment issimilar to the embodiments illustrated in FIGS. 4A and 4B except that abreathing gas supply provides a constant supply of breathing gas, suchas oxygen or an oxygen mixture, to the interior of mask 232. Thisembodiment of the present invention is particularly advantageous in thatit permits a wide variety of diagnostic information to be garnered fromthe patient while the patient is being provided with a breathing gas,which is a common medical procedure.

Mask 232 in FIG. 17 includes a first port 234 into which breathing gasfrom a suitable supply, such as an oxygen tank 233 or oxygenconcentrator, is supplied and a second port 236 that communicates asensor 238 to the interior portion of the mask. It is to be understoodthat the breathing gas need not be directly provided to the userinterface, as shown in FIG. 17. On the contrary, the breathing gas canbe provided to the tube connecting sensor 238 to interface 232, therebyavoiding the need to provide two ports in the mask.

In the illustrated embodiment, sensor 238 corresponds to the circuitryillustrated in FIGS. 5 and 9 of the previous embodiment As with theprevious embodiments, a plurality of holes 240 are provided in the maskso that the mask defines a flow element. It is to be understood,however, that any venting system for communicating the interior of themask with ambient atmosphere, while creating a pressure drop across theflow element, is contemplated by the present invention.

As with the third embodiment illustrated in FIG. 16, the constant supplyof a breathing gas to mask 232 produces a substantially continuous leakfrom the mask. This supply of gas will skew the signals output from thesensor or from the processor so that these signal do not fluctuate aboutzero during the patient's breathing cycle. Instead, these signals willhave a bias that corresponds to the flow of breathing gas into the maskand hence, the leak from the mask. As in the previous embodiment, thepresent invention compensates for this bias, for example, by subtractingthe known leak from the signal output by the sensor or processor. Ofcourse, any other technique for correcting the signals output from thesensor or processor to account for this leak are also contemplated bythe present invention. For example, the vertical axis in the waveformdiagram for the patient's quantitative flow can be re-labeled so thatthe bias level caused by the leak is defines as the effective zero flowaxis. The flow signal will fluctuate about this effective zero flow axisif a constant supply of gas is delivered to the mask.

FIG. 18 illustrates a fifth embodiment of an interface meter accordingto the principles of the present invention. This embodiment is similarto the embodiment discussed above with respect to FIG. 17 except that apositive pressure device 244 supplies a breathing gas to an interface246 via a breathing circuit 248. In the illustrated embodiment,interface 246 is a mask interface that covers the user's nose or theuser's nose and mouth. There are no holes in the mask to serve as aventing element. Instead, an adapter device 250 is coupled to the mask.Adapter device 250 attaches an end of breathing circuit 248 to mask 246.Adapter device 250 also includes at least one hole 252, which can have avariety of configurations, that communicates the interior portion ofmask 246 to the ambient atmosphere. A hollow tube 254 is coupled to aport defined in adapter device 250 to communicate an a sensor 256 withthe interior of mask 246. Sensor 256 performs the same function as thecircuit illustrated in FIGS. 5 and 9. It is to be understood, however,that sensor 256 can be coupled to other portions of the mask a breathingcircuit. For example, sensor 256 can be coupled directly to a pick-offport defined in mask 246 or can be provided along breathing circuit 248,so long as sensor 256 measures a fluid characteristic associated withthe pressure differential caused by venting the interior of the mask toambient atmosphere.

The present invention also contemplates that holes 252 can be removedfrom the interface and/or breathing circuit so that there is no ventingelement between the positive pressure device and the patient. Instead,the gas inlet to the positive pressure device serves as the primaryventing element, i.e., gas inlettout, for the patient circuit. Duringinhalation, the patient's inhalation and the pressure provided by thepositive pressure provide breathing gas to the patient. Duringexhalation, the force of the patient's expiration causes gas to bebacked up into the positive pressure device and out of the gas inletprovided thereon.

As with the third and fourth embodiments illustrated in FIGS. 16 and 17,the constant supply of a breathing gas to mask 246 produces asubstantially continuous leak from the mask via holes 252. As in theprevious embodiments, the present invention compensates for the biascaused by this supply of gas, for example, by subtracting the known leakfrom the signal output by the sensor or processor. If bi-level pressureor variable pressure is provided by positive pressure device 244,compensations techniques such as those discussed above, can be employedto correct for the bias imposed by the variable pressure.

Although FIGS. 17 and 18 illustrate providing a supply of breathing gasto a mask-type patient interface, it is to be understood that abreathing gas, such as oxygen, can be supplied to other types of patientinterfaces, in addition to the incubation chamber illustrated in FIG.15, according to the principles of the present invention. FIG. 19, forexample, illustrates a nasal prong patient interface that is similar tothat illustrated in FIG. 15 except that nasal prong interface 260 inFIG. 19 includes a supply of oxygen to the patient. In all otherrespects, the sixth embodiment of the present invention and theembodiment illustrated in FIG. 14 are the same.

In the illustrated embodiment, nasal prongs 260 include protrudingportions 262 that insert into the nares of the user and opens areprovided in each end of the protruding portions. The proximal end ofprotruding portions 262 include at least one vent hole 264 that performthe same function as vent holes 212 in the nasal cannula illustrated inFIG. 15. A sensor (not shown) that performs the same function as sensor78 in FIGS. 4A and 4B is coupled to the interior portion of bothprotruding portions 262 via a first hollow tube 266 and short,connecting tubes 268. A breathing gas, such as oxygen, is provided tothe interior of protruding portions 262 via a second hollow tube 270 andshort, connecting tubes 272. The constant supply of breathing gas tonasal prongs 262 produces a substantially continuous leak from theprotruding portions holes 264. As in the previous embodiments, thepresent invention compensates for the bias caused by this supply of gas,for example, by subtracting the known leak from the signal output by thesensor or processor.

The present invention also contemplates that a breathing gas can beprovided to the tubing connecting the nasal prong interface to thesensor. This embodiment is advantageous in that it eliminates that needfor two hollow tubes and two connecting tubes to be connected to eachprotruding portion of the nasal prong interface.

Although the invention has been described in detail for the purpose ofillustration based on what is currently considered to be the mostpractical and preferred embodiments, it is to be understood that suchdetail is solely for that purpose and that the invention is not limitedto the disclosed embodiments, but on the contrary, is intended to covermodifications and equivalent arrangements that are within the spirit andscope of the appended claims. For example, while processor 92 and 146have be described in terms of an integrated circuit that is carries outa predetermined program, it is to be understood that these functionscould be accomplished using hardwired circuit elements.

What is claimed is:
 1. A patient monitoring apparatus, comprising: amask adapted to be donned by a user and having an outer user contactingsurface adapted to encircle at least one of a user's nares, a user'smouth, and both a user's nares and mouth such that an interior portionof the mask is in fluid communication with an airway of a user toreceive substantially all gas inhaled and exhaled by such a user; anexhaust vent associated with the mask and communicating the interiorportion of the mask with an ambient atmosphere outside the mask, theexhaust vent and the mask defining a flow element across which apressure differential is created during inhalation and exhalation, thepressure differential being a pressure difference between a firstpressure within the interior portion of the mask and a pressure ofambient atmosphere outside the mask; a sensor communicating with theinterior portion of the mask to measure a fluid characteristic resultingfrom such a pressure differential and to output a first signalindicative of the fluid characteristic; and a processing unit receivingthe first signal and determining a quantitative value for aphysiological characteristic of a user based on the first signal.
 2. Anapparatus according to claim 1, wherein the sensor is coupled to themask via a single hollow tube, wherein at least one of the sensor andthe mask is selectively detachable from the hollow tube.
 3. An apparatusaccording to claim 1, wherein the physiological characteristic is acharacteristic associated with respiration.
 4. An apparatus according toclaim 3, wherein the characteristic associated with respiration is atleast one of a rate of flow of gas through the mask and a volume of gasexiting the interior portion of the mask over at least one of apredetermined period of time and a predetermined portion of arespiratory cycle.
 5. An apparatus according to claim 1, wherein theprocessing unit outputs a second signal indicative of the quantitativevalue, and wherein the apparatus further comprises an output device thatconverts the second signal into a human perceivable output.
 6. Anapparatus according to claim 1, wherein the sensor is a gas flow sensorand the fluid characteristic measured by the sensor is a rate of flow ofgas through the gas flow sensor between the interior portion of the maskand anbient atmosphere.
 7. An apparatus according to claim 1, whereinthe sensor is a pressure sensor and the fluid characteristic measured bythe sensor is a pressure within the interior portion of the mask.
 8. Anapparatus according to claim 1, wherein the exhaust vent is a fixeddiameter hole defined in the mask.
 9. An apparatus according to claim 1,further comprising a breathing gas supply communicating with theinterior portion of the mask to provide breathing gas to the interiorportion of the mask.
 10. An apparatus according to claim 9, furthercomprising means for accounting for an offset in the first signal causedby the supply of breathing gas to the interior portion of the mask bythe gas supply.
 11. An apparatus according to claim 1, wherein theexhaust vent is defined in the mask.
 12. An apparatus according to claim1, further comprising a breathing circuit operatively coupled to themask, and wherein the exhaust vent is defined in the breathing circuit.13. A patient monitoring apparatus, comprising: a nasal cannula havingat least one prong that inserts into a nare of a user such that aninterior portion of the prong is in fluid communication with an airwayof such a user to receive substantially all gas inhaled and exhaled bysuch a user; an exhaust vent associated with the prong and communicatingthe interior portion of the prong with ambient atmosphere, the exhaustvent and the prong defining a flow element across which a pressuredifferential is created during inhalation and exhalation, the pressuredifferential being a pressure difference between a first pressure withinthe interior portion of the prong and a pressure of ambient atmosphere;a sensor communicating with the interior portion of the prong to measurea fluid characteristic resulting from such a pressure differential andto output a first signal indicative of the fluid characteristic; and aprocessing unit receiving the first signal and determining aquantitative value for a physiological characteristic of a user based onthe first signal.
 14. An apparatus according to claim 13, wherein theprong includes an opening defined at a distal end to communicate theinterior portion of the prong with a nasal cavity of the user, andwherein the exhaust vent includes at least one fixed diameter holedefined in a proximal end of the prong.
 15. An apparatus according toclaim 13, wherein the physiological characteristic is at least one of arate of flow of gas through the prong and a volume of gas exiting theinterior portion of the prong over at least one of a predeterminedperiod of time and a predetermined portion of a respiratory cycle. 16.An apparatus according to claim 13, further comprising: a breathing gassupply communicating with the interior portion of the mask to providebreathing gas to the interior portion of the mask; and means foraccounting for an offset in the first signal caused by the supply ofbreathing gas to the interior portion of the prong by the breathing gassupply.
 17. A patient monitoring method, comprising the steps of:donning a mask on a user such that an onter user contacting surface ofthe mask encircles at last one of a user's nares, a user's mouth, andboth a user's nares and mouth and such that an interior portion of themask is in fluid communication with an airway of a user to receivesubstantially all gas inhaled and exhaled by such a user, wherein anexhaust vent is associated with the mask so as to coimunicate theinterior portion of the mask with an ambient atmosphere, the exhaustvent defining a flow element across which a pressure differential iscreated during inhalation and exhalation, the pressure differentialbeing a pressure difference between a first pressure within the interiorportion of the mask and a pressure of ambient atmosphere outside;passing a flow gas through the flow element during at least one ofinhalation and exhalation; measuring a fluid characierstic resultingfrom the pressure differential; outputting a first signal thatcorresponds to the fluid characteristic; and using the first signal todetermine a quantitative value for a physiological characteristic ofsuch a user.
 18. A method according to claim 17, wherein the step ofusing the first signal to determine a quantitative value for thephysiological characteristic includes determining a quantitative valuefor a characteristic associated with respiration.
 19. A method accordingto claim 17, wherein the step of using the first signal to determine aquantitative value for the characteristic associated with respirationincludes determining at least one of a rate of flow of gas through themask and a volume of gas exiting the interior portion of the mask overat least one of a predetermined period of time and a predeterminedportion of a respiratory cycle.
 20. A method according to claim 17,further comprising outputting, in a human perceivable manner, thequantitative value for the physiological characteristic of such a user.21. A method according to claim 17, wherein the measuring step isaccomplished using a gas flow sensor, and the fluid characteristicmeasured during the measuring step is a rate of flow of gas through thegas flow sensor between the interior portion of the mask and ambientatmosphere.
 22. A method according to claim 17, wherein the measuringstep is accomplished using a pressure sensor, and the fluidcharacteristic measured by the pressure sensor is a pressure within theinterior portion of the mask.
 23. A method according to claim 17,further comprising outputting, in a human perceivable manner,information indicative of the physiological characteristic of such auser.
 24. A method according to claim 17, further comprising supplying abreathing gas to the interior portion of the mask.
 25. A methodaccording to claim 24, further comprising accounting for an offset inthe first signal caused by supplying the breathing gas to the interiorportion of the mask by the gas supply.
 26. A method according to claim17, wherein the exhaust vent is defined in the mask.
 27. A methodaccording to claim 17, further comprising a breathing circuitoperatively coupled to the mask, and wherein the exhaust vent is definedin the breathing circuit.
 28. A patient monitoring method, comprisingthe steps of: inserting a nasal prong into a user's nares such that aninterior portion of the prong is in fluid communication with an airwayof a user to receive substantially all gas inhaled and exhaled by such auser, wherein the prong includes an exhaust vent that communicates theinterior portion of the prong with an ambient atmosphere, the exhaustvent and the prong defining a flow element across which a pressuredifferential is created during inhalation and exhalation, the pressuredifferential being a pressure difference between a first pressure withinthe interior portion of the prong and a pressure of ambient atmosphereoutside; passing a flow of gas through the flow element during at leastone of inhalation and exhalation; measuring a fluid characteristicresulting from the pressure differential; outputting a first signal thatcorresponds to the fluid characteristic; and using the first signal todetermine a quantitative value for a physiological characteristic ofsuch a user.
 29. A method according to claim 28, wherein the step ofusing the first signal to determine a quantitative value for thephysiological characteristic includes determining at least one of a rateof flow of gas through the prong and a volume of gas exiting theinterior portion of the prong over at least one of a predeterminedperiod of time and a predetermined portion of a respiratory cycle.
 30. Amethod according to claim 28, further comprising: supplying a breathinggas to the interior portion of the prong; and accounting for an offsetin the first signal caused by supplying the breathing gas to theinterior portion of the prong by the gas supply.